The present invention relates to an improved soda-lime-silica flat glass composition that yields economic advantages in melting and forming, better tempering characteristics, and improved surface durability. The composition is particularly suited to the manufacture of flat glass by the float process.
The composition of commercially produced flat glass has become rather narrowly standardized, with the compositions of flat glass products made by various manufacturers around the world seldom varying more than one or two percentage points of the major constituents from the following typical compositions:
______________________________________ Constituent Weight Percent ______________________________________ SiO.sub.2 73.08 Na.sub.2 O 13.68 K.sub.2 O 0.02 CaO 8.94 MgO 3.88 Al.sub.2 O.sub.3 0.11 SO.sub.3 0.23 Fe.sub.2 O.sub.3 0.12 ______________________________________
Compositions of this type have become standard because they yield a carefully balanced set of properties that are desired for manufacture and subsequent processing of flat glass products. Varying one constituent for the sake of improving one property usually has adverse effects on at least one or more other properties. Some of the properties for which the standard flat glass composition had heretofore been considered optimized include: minimized melting temperature, avoidance of devitrification during forming, surface abrasion resistance, surface weather durability, low refractory attack during melting, temperability, and batch cost. The melting temperature of the standard flat glass composition set forth above is 2630.degree. F. It has long been known that reducing the amount of silica and/or increasing the amount of alkali in the glass can lower the melting temperature of the glass and lower the energy requirements for melting, but doing so undesirably reduces the surface durability of the glass product. Adjusting other constituents to compensate for the loss of durability can result in other drawbacks such as a reduction in the "working range," that is, a reduction in the temperature range in which the glass can be formed without substantial devitrification of the glass. As a result, it has heretofore been considered impractical for flat glass producers, particularly float glass producers, to lower the silica content of the glass significantly below 70 percent in order to obtain the melting advantages and energy savings.
U.S. Pat. No. 3,833,388 (Ohlberg et al.) discloses a flat glass composition having a higher alkali content than usual, but with the silica content no lower than 70 percent. Therefore, the full potential of reducing the melting temperature is not realized by that composition.
U.S. Pat. No. 3,779,733 (Janakirama-Rao) discloses broad compositional ranges for flat glass compositions that include silica concentrations considerably below 70 percent, but does not provide enablement for successfully producing flat glass having a silica concentration lower than 70 percent. The patent deals with producing a glass product having certain transmittance characteristics, not with improving melting properties.
U.S. Pat. No. 2,581,639 (Duncan et al.) discloses a television faceplate glass adapted to be sealed to a metal component and described as being suitable for manufacture by the sheet drawing process. The silica concentration is only slightly below 70 percent.
U.S. Pat. No. 2,669,808 (Duncan et. al.) discloses another glass composition adapted for sealing to metal components of a television tube. The glass is intended to be made by a sheet drawing process, but the silica concentration is extremely low for the glass to be considered suitable for general flat glass applications such as building or vehicle glazing. The low silica concentration would be expected to result in low surface durability.
Japanese Patent No. 61-197444 discloses soda-lime-silica glass compositions with silica concentration below 70 percent for the purpose of improving the tempering properties of the glass. The lower SiO2 concentrations together with higher than usual CaO concentrations would be expected to yield melting advantages. However, the reliance on increasing the CaO concentration and maintaining relatively moderate amounts of alkali result in undesirable flattening of the temperature/viscosity curve, so that temperatures at the viscosity range suitable for forming into a flat ribbon are increased and the danger of devitrification is increased (i.e. the working range is decreased). For some float forming operations, some of the examples disclosed in the Japanese patent could not be formed without devitrification or would require considerable cooling of the glass between the melting and forming stages. Although the patent discloses a fairly broad range of alumina, attainment of the desired results appears to require a relatively large amount of alumina (5%) in the examples or a combination of alumina and titania, both of which add substantially to the batch cost. A relatively high total of silica and alumina in the examples also indicates that reduction of the melting temperature was not optimized.
A general discussion of soda-lime-silica glasses, their constituents, and the relationship between the constituents and some of the properties of the glass products can be found in The Properties of Glass by G. W. Morey (Reinhold, 1954) pages 74-78. Of the several examples of glass compositions given there, none of the examples having less than 70 percent silica are mass-produced flat glass. Example 1 at 60 percent silica is described as lacking in durability.